Phased arrays are used in a wide variety of applications. For example, radar phased arrays are used in military aircraft, naval ships, military satellites and drone systems to detect, jam, and for missile guiding purposes. Phased arrays are also used in a large variety of non-military applications including, for example, air and terrestrial traffic detection and control systems, radio broadcasting, earth-orbiting satellites, space probe communications, cellular systems, and weather research and forecasting systems. Unfortunately, a single multi-purpose phased array system that can be used for all of these different types of applications and which can be used for both radar and communications applications, in particular, is not currently available.
A phased array includes a number of spatially separated but proximate antenna elements. The number of antenna elements used depends on the application. In principle, a phased array can be made with as few as two antenna elements. However, the number that is used is usually much greater. For example, the FPS-85 radar system in Eglin, Fla. and operated by the United States Air Force has over 5,000 transmitter antenna elements and nearly 20,000 receive antenna elements.
FIG. 1 is a drawing showing the basic parts of a conventional phased array system 100. The phased array system 100 includes a plurality of transmit-receive (TR) modules, a plurality of associated antenna elements 104, and a beamformer 106. A high-frequency radio frequency (RF) source 108 is also provided, which directs low-power, RF signals into the transmit paths of the TR modules 102 during times the TR modules 102 are transmitting, and a receiver 110 for processing received RF signals.
The transmit path in each TR module 102 includes a linear (e.g., Class A, B, or AB) high-power amplifier (HPA) 112. The receive path in each TR module 102 includes a low-noise amplifier (LNA) 114. The antenna element 104 associated with each TR module 104 is used for both transmitting and receiving. When transmitting, low-power RF signals from the RF source 108 are directed through the beamformer 106 and amplified into high-power RF transmit signals by the HPAs 112. The high-power RF transmit signals from each TR module 102 are then directed to the antenna elements 104, which convert the high-power RF transmit signals into high power electromagnetic waves and radiate the high-power electromagnetic waves out over the air or into space where they are received by a remote target. When receiving, the antenna elements 104 of the phased array system 100 capture electromagnetic energy from incident electromagnetic waves and convert the captured electromagnetic energy into RF electrical signals. Because the electromagnetic energy captured by the antenna elements 104 is usually very weak, the received RF electrical signals must be amplified by the LNAs 114 before they can be further processed downstream. The LNAs 114 are designed for low noise figure and are incapable of handling high input powers. Accordingly, to protect the LNAs 114 from being damaged and prevent receiver desensitization, the received RF signals are first passed through limiters 116 before being directed into the inputs of the LNAs 114.
The antenna elements 104 in phased array transceiver systems are configured in close proximity so that that the RF power radiated by the antenna elements 104 during transmission can constructively interfere and combine to form a “beam,” and so that the very weak signals impinging on the antenna array can be more readily detected. The beamformer 106 controls the phase relationships among the transmitted RF signals in the transmit paths of the TR modules 102 and, consequently, the direction of transmission (or “beam angle”) of the transmitted beam. Depending on the application, the beamformer 106 is configured to establish a beam with a fixed beam angle or a beam angle that is variable or adaptive. In applications requiring a fixed beam angle, such as in a tower array or a geostationary satellite, the antenna array is aimed in the desired direction and the phase relationships among the transmitted RF signals in the transmit paths of the TR modules 102 are set and fixed to achieve the required fixed beam angle and then not adjusted thereafter. In applications requiring a variable or adaptive beam angle, such as in radar applications where a target may be moving, the phase relationships of the transmitted RF signals must be varied. To accomplish this, the beamformer 106 adjusts and controls the relative time delays or relative phase shifts of the transmitted RF signals. By individually adjusting and controlling the time delays or phase shifts, the transmit beam angle can be varied or, in other words, the transmit beam can be “steered.” The beamformer 106 may be further configured to control the relative time delays or relative phase shifts of the received RF signals passing through the receive paths of the TR modules 102. Individually adjusting and controlling the time delays or phase shifts of the received signals allows the receive array pattern to be adjusted to a desired or required receive array pattern.
One serious and well-known problem associated with TR modules is that the RF power generated by the TR module's HPA can be reflected by the TR module's antenna element and back into the output of the HPA, instead of being fully radiated by the antenna element. The reflected transmit RF power is highly undesirable since it can alter the HPA's load impedance and contribute to intermodulation distortion. Transmitted power can also be undesirably reflected from the antenna element and into the receive path of the TR module, causing distortion of the RF signals being received by the TR module.
Another serious and well-known problem is that when the TR module 102 is configured in an array, along with other TR modules (as in FIG. 1), RF signals transmitted from an antenna element of one TR module can be undesirably intercepted by antenna elements of other TR modules in the array. These “reverse” signals are also highly undesirable since they can be passed into the intercepting modules' transmit and receive paths and cause further distortion in the transmitted and received signals.
In an effort to address these problems, circulators 118 are employed in the TR modules 102 of conventional phased array systems. As shown in FIG. 1, each of the TR modules 102 is equipped with its own circulator 118, and is a three-port device having a first port connected to the output of its associated TR module's HPA 112 (transmit path port), a second port connected to the TR module's antenna element 104 (antenna port), and a third port connected to the input of the receive path of the TR module 102 (receive path port). The directional properties of the circulator 118 are asymmetric (i.e., are non-reciprocal). This asymmetry is exploited and relied on in conventional TR module 102 to prevent reflected transmit signals and reverse signals from other TR modules from being directed back into the transmit path and into the output of the HPA 112. Specifically when the TR module is transmitting, its circulator 118 provides a low impedance path for signals directed from the circulator's transmit path port to its antenna port, thereby allowing transmitting to occur, but isolates the transmit port from the antenna port in the reverse direction, for example, by attenuating signals (such as reflected signals or reverse signals) flowing in the reverse direction.
Although the circulators 118 can be effective, they do not provide any protection for signals that are reflected into the receive path of the TR module and do not provide protection against reverse signals from other TR modules in the array from being directed into the receive path. Furthermore, the circulators 118 are effective at preventing transmit reflected signals and reverse signals into the transmit path only over a very narrow range of operating frequencies. This narrowband limitation is highlighted in FIGS. 2A and 2B, which are scattering parameter measurements taken on a typical circulator. FIG. 2A shows the forward transfer coefficient (scattering parameter (S-parameter) S21) of the circulator swept over a 900 MHz to 1 GHz frequency range. FIG. 2B shows the reverse transfer coefficient (S-parameter S12) of the circulator swept over the same frequency range. As can be seen in FIG. 2A, the forward transfer coefficient S21 remains flat and near 0 dB (˜−0.27 dB) over the entire swept frequency range, indicating that for the forward direction, if the circulator was to serve as one of the circulators 118 in one of the TR modules 102 of the phased array system 100 in FIG. 1, it would be effective at delivering most of the power from port 1 (attached to transmit path port) to port 2 (attached to antenna port). However, FIG. 2B reveals that the reverse transfer coefficient S12 provides high isolation in the reverse direction only over a very narrow isolation band of ˜30 MHz. The very narrow isolation band means that if the circulator was to serve as one of the circulators 118 in one of the TR modules 102 of the phased array system 100 in FIG. 1, transmitted RF signals outside the isolation band would be susceptible to being reflected by the TR module's antenna element 104 back into the transmit path of the TR module 102.
Circulators must be used in conventional phased array systems in order to prevent transmit signals and reverse signals from reflecting back into the transmit paths of the TR modules 102. However, the presence of the circulators 118 and their narrowband limitations precludes the conventional phased array system 100 from being used in any application except for the specific application for which it is designed. In other words, conventional phased array systems are not multi-purpose and cannot be used for multiple applications, such as for both radar and communications applications, for example.
In addition to the narrowband restrictions imposed by the circulators 118, the circulator 118 does not do anything to prevent reflected transmit signals from being directed into the receive path of a TR module 102, and does not do anything to prevent reverse signals from other TR modules 102 from being directed into TR module's receive path. The receive signals are therefore susceptible to being distorted by transmit reflected signals and reverse signals from other TR modules.
Other drawbacks associated with circulators are that they are large passive devices that have insertion losses, consume power, occupy large areas of the printed circuit board (PCB) on which they and the other components of the TR modules 102 are formed, and contribute to the overall weight and size of the phased array system 100. As can be seen in FIG. 3, which is a photograph of a typical conventional TR module 300, the circulator 304 and associated PCB waveguide traces occupy nearly a third of the area of the PCB 302. Moreover, because the HPA 112 of the TR modules 102 is a linear amplifier, it is large in size and very inefficient. Due to its inefficiency, a very large heatsink 306 (see FIG. 3) is required to conduct heat away from the HPA 112 and to protect the HPA 112 from being damaged, and a larger power supply than desired is necessary to compensate for the HPA's inefficiency. The large circulator 304 and large heatsink 306 also add to the cost, size and weight of the TR module 102. Because phased array systems will often include hundreds and sometimes thousands of TR modules 102, the incremental cost, size and weight of each TR module 102 must be multiplied by hundreds or thousands of times in determining the overall cost, size and weight of the entire system. Furthermore, with hundreds and possibly thousands of very inefficient HPAs 112, large and heavy power supplies are required to compensate for the multiple inefficiencies and very large and heavy cooling systems are necessary to displace the enormous amount of heat generated by the hundreds and possibly thousands of HPAs 112.